Robert is the son of Hazel Alvera (Boe) and Robert John Wagner. He is of half German and half Norwegian descent.

Robert is married to actress Jill St. John. He was previously married to actress Marion Marshall, and to actress Natalie Wood, until her death. He has two children, a daughter, television personality Katie Wagner, with Marion, and a daughter, Courtney, with Natalie.

Robert’s paternal grandfather was named Matthias/Matthew Wagner. Matthias was German.

Robert’s maternal grandfather was Anders “Andrew” Hansson Løkøien/Bo/Boe (the son of Hans Knutsson/Knutsen Løkøien and Marit Olsdotter/Olsen Skrinde). Andrew was born in Lom, Oppland, Norway. Hans was the son of Knut Hansson Løkøien/Bøe/Aukrustbakken and Synne Andersdotter Kvamme. Marit was the daughter of Ola Olsson Ramstad/Hørven and Anne Sylfestdotter Skrinde.